1. Field of the Invention
The invention relates to low bobbin thread detection systems for a sewing machine. 2. Description of the Prior Art
It is well known to provide a sewing machine with a low bobbin thread detection system including a light source on one side of a loop taker to project light rays along a path in close proximity to the hub of a bobbin, and further including a photodetector on the other side of the loop taker to detect light rays from the light source when thread on the bobbin is about to be depleted and thereupon initiate the operation of a signaling device to warn the machine operator of the impending exhaustion of the bobbin thread. A system of the described type is disclosed in the copending application by K. D. Adams for "Adjustable Bobbin Thread Run-Out Indicator", Ser. No. 916,614, filed June 19, 1978 and assigned to The Singer Company. In such systems, a photodetector is disposed in a holder, and an aperture provided in the holder to permit light rays generated by the light source to enter the holder and fall upon the photodetector. Unfortunately, however, lint tends to accumulate in the aperture and adversely affect the operating reliability of the low bobbin thread detection system. The reliability is also deleteriously affected by external light such as may pass, for example, through a bobbing thread window, and thence through the aperture to impinge upon the photodetector.
It is an object of this invention to provide an optical shield over the aperture of the photodetector holder of a low bobbin detector system of the kind described to prevent the accumulation of lint in the aperture, and it is a further object to have such shield focus light from the system's light source upon the photodetector to thereby increase the signal to noise ratio due to light generated by said source and to external light respectively so as to enable the design of a system of greater reliability than would otherwise be possible.